1. Field of the Invention
The present disclosure relates to an organic light emitting display device and a manufacturing method thereof, and particularly, to a top emission type organic light emitting display device and a manufacturing method thereof.
2. Background of the Invention
Recently, interest in information displays has been on the rise, demand for using portable information mediums has grown, and research into and commercialization of lighter, thinner flat panel displays (FPDs) has actively conducted.
In the flat panel display field, liquid crystal displays (LCDs) which are light in weight and consume less power have come to prominence.
Among display devices, an organic light emitting display device is self-luminous and thus it is excellent in terms of a viewing angle and a contrast ratio, compared with an LCD. Also, since the organic light emitting display device does not require a backlight, the organic light emitting display device may be lighter and thinner and is advantageous in terms of power consumption. In addition, the organic light emitting display device may be driven with a low DC voltage and may have a fast response speed.
Hereinafter, a basic structure and operational characteristics of an organic light emitting display device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating a light emitting principle of a general organic light emitting diode.
In general, an organic light emitting display device includes an organic light emitting diode (OLED) as illustrated in FIG. 1.
Referring to FIG. 1, an OLED includes an anode 18, a pixel electrode, a cathode 28, a common electrode, and organic compound layers 31, 32, 35, 36, and 37 formed between the anode 18 and the cathode 28.
Here, the organic compound layers 31, 32, 35, 36, and 37 include a hole injection layer 31, a hole transport layer 32, an emission layer 35, an electron transport layer 36, and an electron injection layer 37.
In the OLED configured thusly, when a positive (+) voltage and a negative (−) voltage are applied to the anode 18 and the cathode 28, respectively, holes passing through the hole transport layer 30b and electrons passing through the electron transport layer 30d are transferred to the emission layer 30c to form excitons, and when the excitons transition from an excited state to a ground state, namely, a stable state, to thus emit light.
In the organic light emitting display device, subpixels each including the OLED having the foregoing structure are arranged in a matrix form and selectively controlled with a data voltage and a scan voltage to display various colors that collectively form an image.
Here, the organic light emitting display device can be categorized into a passive matrix organic light emitting display device and an active matrix type organic light emitting display device using a thin film transistor (TFT) as a switching element. In the active matrix type organic light emitting display device, a TFT, an active element, is selectively turned on to select a subpixel and maintain light emission of the subpixel due to voltage charged in a storage capacitor.
Also, the organic light emitting display device having the foregoing subpixel structure may be implemented as a top emission type organic light emitting display device, a bottom emission type organic light emitting display device, or a dual-emission type organic light emitting display device according to directions in which light is emitted.
In the top emission type organic light emitting device emits light in a direction opposite to a substrate on which subpixels are arranged. The top emission type organic light emitting device is advantageous in that an aperture ratio is greater than that of the bottom emission type organic light emitting device in which light is emitted in a direction toward the substrate in which subpixels are arranged.
In the top emission type organic light emitting device, an anode is formed below an organic compound layer, and a cathode is formed above the organic compound layer in which light is transmitted.
Here, the cathode should be formed to be thin enough so as to be implemented as a translucent film having a low work function. However, doing so causes the cathode to have high resistance.
Thus, in the top emission type organic light emitting display device, voltage drop (IR drop) occurs due to the high resistivity of the thin structured cathode. Thus, voltages having different levels are applied to subpixels, causing non-uniformity of luminance or image quality. In particular, as the size of a display panel increases, the voltage drop problem may be aggravated.